fix: default bridge and tests to project venv Python

Use .venv/bin/python3 by default, with PYTHON_PATH as an override, and update the setup script and docs to match the new virtualenv-based workflow.

tests/integration/pykeepass.test.ts

@@ -26,7 +26,8 @@ const FIXTURE_PATH = "tests/fixtures/data.kdbx";
 const FIXTURE_DATA_PATH = "tests/fixtures/data.kdbx.json";
 
 async function ensurePyKeePass(): Promise<boolean> {
-	const child = Bun.spawn(["python3", "-c", "import pykeepass; print('ok')"], {
+	const python = process.env.PYTHON_PATH ?? ".venv/bin/python3";
+	const child = Bun.spawn([python, "-c", "import pykeepass; print('ok')"], {
 		stdout: "pipe",
 		stderr: "pipe",
 	});
@@ -95,7 +96,7 @@ test("lists the groups from the bundled data fixture", async () => {
 	}
 
 	const db = openKeePassDatabase(FIXTURE_PATH, { password });
-const groups = await db.listGroups();
+	const groups = await db.listGroups();
 	expect(groups).toEqual([
 		{ name: "Racine", path: "" },
 		{ name: "Folder1", path: "Folder1" },
@@ -145,4 +146,3 @@ test("uses the JSON fixture content as the source of truth for expectations", as
 		"\t\t- f2-item1",
 	]);
 });
-